This invention relates to a toner for the development of an electrostatic image for use in electrophotography and electrostatic latent image developers, more specifically it relates to a toner for the development of an electrostatic image which is excellent in blocking resistance and offset resistance as well as excellent fixability, particularly low temperature fixability, and a developer for developing an electrostatic latent image in electrophotography, electrostatic recording and electrostatic printing.
Generally speaking, in the dry system developing method, a powdery toner for development of electrostatic image charged is attached through electrical attraction onto an electrostatic latent image on a photosensitive member, then transferred onto a paper and fixed by hot rolls, etc.
Accordingly, such toner for development of electrostatic image is demanded to have various properties such as anti-blocking property (toner particles should not be aggregated), anti-offset property (toner should be firmly attached to paper). Particularly, recently, the toner for development of electrostatic image has been demanded to have good fixability at lower temperatures. In the prior art, for improvement of anti-offset property, it has been proposed to constitute the binder in the toner for development of electrostatic image of a resin comprising a polymer component with lower molecular weight and a polymer component with higher molecular weight (see Japanese Provisional Patent Publications No. 158340/1981, No. 16144/1981 and No. 202455/1983).
However, although an improvement of anti-offset property can be surely accomplished by introducing a polymer component with higher molecular weight into the toner for development of electrostatic image, if the glass transition point or softening point of the lower molecular weight polymer component is lowered in order to realize good fixability at lower temperature without causing lowering of the improved anti-offset property, anti-blocking property will be lowered.
On the other hand, if the proportion of the polymer component with lower molecular weight is increased by decreasing the proportion of the polymer component with higher molecular weight in order to realize fixability at lower temperature, anti-offset property will be worsened.
On the other hand, there is also a proposal, in which the binder is constituted of, for example, a styrene-acrylic acid type polymer, and its molecular weight distribution is made broader without particularly incorporating a polymer component with higher molecular weight as described above in this polymer, and moreover, by forming ion bonding of the carboxyl group in the polymer with metal atoms by the reaction with a metal compound to form a cross-linked structure through metal atoms, thus consequently realizing improvement of anti-offset property by effecting formation of substantially high polymer through the cross-linked structure, although having no special polymer component with higher molecular weight (see Japanese Provisional Patent Publications No. 110155/1981 and No. 110156/1981).
However, such toner for development of electrostatic image contains a large amount of a metal compound formulated therein, and therefore the metal compound formulated may exhibit a catalytic action depending on the condition, whereby the resin in the toner for development of electrostatic image may be readily gelled, and therefore there are problems such that it is difficult to determine preparation conditions for obtaining a desired toner for development of a metal compound, or that even the preparation condition can be determined, reproducibility is bad, etc.
In an electrophotographic method, after charging uniform surface charge to a photoreceptor comprising a photoconductive element in a dark place, by carrying out a development to form an electrostatic latent image, and thereafter the electrostatic latent image is developed to form a visual image.
Generally, such methods to develop an electrostatic latent image (hereinafter abbreviated to as "latent image") can be roughly classified into a wet development and a dry development. The wet development is a method which employs a liquid developer comprising dispersing various pigments or dyes in insulating organic liquids as fine particles, and the dry development is a method in which development is carried out by using a fine particle detecting powder called a toner which comprises dispersing a colorant such as a carbon black in a natural or synthetic resin. In the dry development, there are a so-called hair brush method, impression method and powder cloud method where a development is carried out by using a toner as a main component, and a so-called magnetic brush method and cascade method where a mixture of a toner and a carrier comprising a iron powder or glass beads is used as a developer.
By these developing method, toner particles having charges in the developer adhere to a latent image whereby a visual image is formed. This visual image is transferred onto a photoreceptor as it were by a pressure or a solvent vapor or to another support such as a transfer paper and thereafter fixed thereon.
Generally, a carrier constituting a developer can be roughly classified into a conductive carrier and an insulating carrier.
As the conductive carrier, oxidized or unoxidized iron powder has usually been employed. In the developer using the iron carrier, there are disadvantages that friction chargeability to the toner is unstable and fog is occurred in a visualized image obtained. That is, during a friction with the toner, the toner substance will likely adhere on the surface of the iron carrier particles whereby electric resistance of the carrier particles increase and bias current which is usually applied during development is lowered. And yet, friction chargeability of the carrier becomes unstable and as a result, image density of the visualized image formed is lowered and fog increases. Accordingly, when forming of image is continuously repeated with an electrophotographic device by using a developer employed the iron carrier, the developer will deteriorate within a little time and thus it is necessary to change the developer at earlier state whereby good image cannot be obtained continuously.
As the insulating carrier, in general, a carrier in which a surface of a carrier core material comprising a ferromagnetic substance such as iron, nickel, ferrite, etc. is uniformly coated by an insulating resin is representative. In the developer using the insulating carrier, to fuse the toner substance on the surface of the carrier particles is less as compared with the case of the conductive carrier.
However, even in the developer comprising the insulating carrier and the toner, stable friction chargeability is not sometimes obtained. That is, in the developer comprising the toner and the carrier, friction contact between the carrier particles with each other, the toner particles with each other, the carrier particles and toner particle, and the carrier particles and/or the toner particles and a stirring membrane or a device wall in the developing devices, when the coating layer of the carrier is likely to cause wear out due to friction, stability of the friction chargeability of the carrier becomes lost. Further, when the toner particles is easily broken by mechanical impact or pressure, fog will likely be caused based on fine powder and when the toner particles is likely to cause wear out due to friction, friction chargeability of the carrier becomes unstable since the toner substance is likely adhere to the surface of the coating layer of the carrier.
Further, in friction contact between constituting particles of the developer, when fluidity of the developer is not good by restriction of freely movement of the particles due to large contact resistance, unevenness in sprinkle concentration is caused at the sleeve surface for developer transfer whereby development uniformity is lost and image quality becomes low. Also, since friction charge becomes insufficient, clear image with high density cannot be obtained.
Moreover, while a carrier in which the surface of a core material of a carrier is coated by a thermoplastic resin or a cross-linking resin has been developed, in this technique, since only characteristics of the toner is in question, stable friction chargeability cannot be obtained due to defect of the toner combinedly used. As a result, it has a disadvantage that good image without fog cannot be formed for many number of times.
Also, in recent years, from the demands of (a) to restrain overheating and deterioration of a copying machine, (b) to prevent deterioration due to heat of a photoreceptor, (c) to shorten warm up time required for elevating a temperature of a thermal roll, which is capable of fixing, from starting a fixing device, (d) to make little lowering of temperature of a thermal roll due to absorption of heat by a transfer paper whereby to enable a continuous copying over many times and (e) to heighten thermal stability, it is strongly required to enable to carry out the fixing processing in a state of lower temperature of the thermal roller by decreasing a consumption electric power. Accordingly, in the toner, it is required to well fixable even in a lower temperature.
And yet, in the toner, it is necessary to be stably exist without causing aggregation in the condition of usage or storage circumstances, that is, it should be excellent in blocking resistance. Further, in the thermal roller system which is preferred as the fixing method, an offset phenomenon, i.e., a phenomenon of which a part of the toner constituting an image at fixing transfers to a surface of the thermal roller and this is retransferred to a next sent transfer paper to stain an image, is likely caused. Thus, it is required to provide a characteristic of preventing the offset phenomenon, i.e., offset resistance to the toner. In order to obtain the above characteristics, various investigations have been carried out but those which can satisfy all the characteristics could not be obtained.
For example, in order to improve offset resistance, there is a proposal to constitute a resin to be used from a lower molecular weight polymer component and a higher molecular weight polymer component (Japanese Provisional Patent Publications No. 158340/1981, No. 16144/1981 and No. 202455/1983). However, when the high molecular weight polymer component is incorporated into the toner, offset generating temperature becomes high whereby it is convenient for offset resistance, but fixable temperature also increases at the same time so that it causes inconvenience for lower temperature fixing. To the contrary, in order to realize a fixing at lower temperature, if a glass transition temperature or softening temperature of the resin is lowered, not only the offset resistance becomes bad but also it provides defects of lowering in blocking resistance, lowering durability due to filming of the toner to the carrier particles, or generation of filming of carrier particles or a surface of the photoreceptor at high temperature and high humidity conditions.
Also, for example, in Japanese Provisional Patent Publications No. 178250/1982, No. 110155/1986 and No. 110156/1986, there are attempts to increasing offset resistance, blocking resistance and fixing temperature tolerance by a resin obtained by reacting a polymer having a carboxy group and a metal compound. However, the one component developer or the two components developer using an iron powdery carrier disclosed in the above publications is unstable in friction chargeability under high temperature and high humidity conditions. Also, it tends to cause filming to the carrier particles or a surface of the photoreceptor, and cleaning defects are caused due to fusing to a cleaning blade whereby good image cannot be obtained continuously and durability is lowered.